The present invention relates to magnet powder and a sintered magnet comprising a hexagonal ferrite, a bonded magnet and a magnetic recording medium comprising the magnet powder, and a magnetic recording medium having a thin film magnetic layer comprising a hexagonal ferrite phase.
As a material for an oxide permanent magnet, a magnetoplumbite (M type) hexagonal strontium (Sr) ferrite or barium (Ba) ferrite has been mainly used. Calcium (Ca), which is one of the alkaline earth elements as similar to Ba and Sr, has not been used as a magnet material though it is not expensive because Ca does not form a hexagonal ferrite.
In general, a Ca ferrite has a stable structure of CaO-Fe2O3 or CaO-2Fe2O3, and does not form a hexagonal ferrite (CaO-6Fe2O3), but it has been known that a hexagonal ferrite is formed by adding La. In this case, it is considered that the valence of a part of the Fe ion is changed (Fe3+ to Fe2+) to compensate the difference in valence between La and Sr (La3+ and Sr2+). However, the magnetic characteristics obtained in this case is those equivalent to a Ba ferrite at most, which is not considerably high. Furthermore, there has been no example in that an element forming a divalent ion and La are complexly added to a Ca ferrite.
What are important among characteristics of a magnet are a residual magnetic flux density (Br) and an intrinsic coercive force (HcJ).
Br is determined by the density of the magnet, the degree of orientation of the magnet, and the saturation magnetization (4xcfx80Is) determined by the crystal structure. Br is expressed by the following equation:
Br=4xcfx80Isxc3x97(degree of orientation)xc3x97(density)
The Sr ferrite and the Ba ferrite of M type has a 4xcfx80Is value of about 4.65 kG. The density and the degree of orientation each is about 98% at most in the sintered magnet, which provides the highest values. Therefore, Br of these magnets is limited to about 4.46 kG at most, and it has been substantially impossible to provide a high Br value of 4.5 kG or more.
The inventor of the invention have found that the addition of appropriate amounts of La and Zn in an M type ferrite raises its 4xcfx80Is value by about 200 G at most, and a Br value of 4.4 kG or more can be obtained, as described in U.S. patent application Ser. No. 08/672,848, now U.S. Pat. No. 5,846,449. In this case, however, since the anisotropic magnetic field (HA), which will be described later, is decreased, it is difficult to obtain a Br value of 4.4 kG or more and an HcJ of 3.5 kOe or more at the same time.
HcJ is in proportion to the product (HAxc3x97fc) of the anisotropic magnetic field (HA (=2K1/Is)) and a single magnetic domain grain fraction (fc), in which K1 represents a crystal magnetic anisotropy constant, which is determined by the crystal structure as similar to Is. The M type Ba ferrite has K1 of 3.3xc3x97106 erg/cm3, and the M type Sr ferrite has K1 of 3.5xc3x97106 erg/cm3. It has been known that the M type Sr ferrite has the largest K1 value, but it has been difficult to further raise the K1 value.
On the other hand, in the case where ferrite grains are in a single magnetic domain condition, the maximum HcJ is expected because the magnetization must be rotated against the anisotropic magnetic field to reverse the magnetization. In order to make ferrite grains into single magnetic domain grains, the size of the ferrite grains must be smaller than the following critical diameter (dc) expressed by the following equation:
dc=2(kxc2x7Tcxc2x7K1/a)xc2xd/Is2
wherein k represents the Boltzman constant, Tc represents a Curie temperature, and a represents a distance between iron ions. In the case of the M type Sr ferrite, since dc is about 1 xcexcm, it is necessary for producing a sintered magnet that the crystal grain size of the sintered magnet must be controlled to 1 xcexcm or less. While it has been difficult to realize such a fine crystal grain and the high density and the high degree of orientation to provide a high Br at the same time, the inventor has proposed a new production process to demonstrate that superior characteristics that cannot be found in the art are obtained, as described in U.S. Pat. No. 5,648,039. In this process, however, the HcJ value becomes 4.0 kOe when the Br value is 4.4 kG, and therefore it has been difficult to obtain a high HcJ of 4.5 kOe or more with maintaining a high Br of 4.4 kG or more at the same time.
In order to control a crystal grain size of a sintered body to 1 xcexcm or less, it is necessary to make the grain size in the molding step 0.5 xcexcm or less with taking the growth of the grains in the sintering step into consideration. The use of such fine grains brings about a problem in that the productivity is generally deteriorated due to increase in molding time and increase in generation of cracks on molding. Thus, it has been very difficult to realize high characteristics and high productivity at the same time.
It has been known that the addition of Al2O3 and Cr2O3 is effective to obtain a high HcJ value. In this case, Al3+ and Cr3+ have effects of increasing HA and suppressing the grain growth by substituting for Fe3+ having an upward spin in the M type structure, so that a high HcJ value of 4.5 kOe or more is obtained. However, when the Is value is reduced, the Br value is considerably reduced since the sintered density is reduced. As a result, the composition exhibiting the maximum HcJ of 4.5 kOe can only provide a Br value of 4.2 kG.
A sintered magnet of the conventional anisotropic M type ferrite has a temperature dependency of HcJ of about +13 Oe/xc2x0 C. and a relatively high temperature coefficient of about from +0.3 to +0.5%/xc2x0 C., which sometimes bring about great reduction in HcJ on the low temperature side and thus demagnetization. In order to prevent such demagnetization, the HcJ value at room temperature must be a large value of about 5 kOe, and therefore it is substantially impossible to obtain a high Br value at the same time. Powder of an isotropic M type ferrite has a temperature dependency of HcJ of at least about +8 Oe/xc2x0 C., although it is superior to the anisotropic sintered magnet, and a temperature coefficient of +0.15%/xc2x0 C., and thus it has been difficult to further improve the temperature characteristics.
The inventors have proposed that the temperature dependency of HcJ is reduced by introducing distortion into ferrite grains by pulverization, as described in U.S. Pat. No. 5,468,039. In this case, however, HcJ at room temperature is also decreased, and thus the high HcJ at room temperature and its temperature characteristics cannot be improved at the same time.
An object of the invention is to realize a hexagonal ferrite having both a high saturation magnetization and a high magnetic anisotropy, so as to provide a ferrite magnet having a high residual magnetic flux density and a high coercive force, which cannot be realized by the conventional hexagonal ferrite magnet.
Another object of the invention is to provide a ferrite magnet excellent in temperature characteristics of the coercive force, where in particular, reduction of the coercive force in a low temperature region is small.
Further object of the invention is to provide a ferrite magnet having a high residual magnetic flux density and a high coercive force by using relatively coarse ferrite grains having a diameter exceeding 1 xcexcm.
Still further object of the invention is to provide a magnetic recording medium having a high residual magnetic flux density and being thermally stable.
The objects of the invention can be attained by one of the constitutions (1) to (13) described below.
(1) An oxide magnetic material comprising a primary phase of a hexagonal ferrite containing Ca, R, Fe and M, where M represents at least one element selected from the group consisting of Co, Ni and Zn, and R represents at least one element selected from the group consisting of Bi and rare earth elements including Y, with La being essentially included in R.
(2) An oxide magnetic material as in item (1), wherein proportions of the metallic elements Ca, R, Fe and M with respect to the total amount of the metallic elements are
from 1 to 13 atomic % for Ca,
from 0.05 to 10 atomic % for R,
from 80 to 95 atomic % for Fe, and
from 1 to 7 atomic % for M.
(3) An oxide magnetic material as in item (1) or (2), wherein proportions of the metallic elements Ca, R, Fe and M is represented by formula (I):
Ca1xe2x88x92xRx(Fe12xe2x88x92yMy)zO19xe2x80x83xe2x80x83(I)
wherein
0.2xe2x89xa6xxe2x89xa60.8,
0.2xe2x89xa6yxe2x89xa61.0, and
0.5xe2x89xa6zxe2x89xa61.2.
(4) An oxide magnetic material as in one of items (1) to (3), wherein a proportion of Co in M is 10 atomic % or more.
(5) Ferrite particles comprising an oxide magnetic material as in one of items (1) to (4).
(6) Ferrite particles as in item (5), wherein the ferrite grains have a temperature dependency of a coercive force xcex94HcJ/xcex94T within the range of from xe2x88x9250 to 50xc2x0 C. is from xe2x88x925 to 5 Oe/xc2x0 C.
(7) A bonded magnet comprising ferrite particles as in item (5) or (6).
(8) A magnetic recording medium comprising ferrite particles as in item (5) or (6).
(9) A sintered magnet comprising an oxide magnetic material as in one of items (1) to (4).
(10) A sintered magnet as in item (9), wherein the sintered magnet has a temperature dependency of a coercive force xcex94HcJ/xcex94T within the range of from xe2x88x9250 to 50xc2x0 C. is from xe2x88x925 to 10 Oe/xc2x0 C.
(11) A sintered magnet as in item (9) or (10), wherein the sintered magnet has an intrinsic coercive force HcJ in terms of kOe and a residual magnetic flux density Br in terms of kG satisfying the following conditions at 25xc2x0 C.:
Br+⅓HcJxe2x89xa75.75 where HcJxe2x89xa74xe2x80x83xe2x80x83(IV)
Br+{fraction (1/10)}HcJxe2x89xa74.82 where HcJ less than 4xe2x80x83xe2x80x83(V)
(12) A process for producing ferrite grains as in item (5) or (6), wherein calcination or sintering is conducted in an atmosphere having an oxygen partial pressure of more than 0.05 atm.
(13) A process for producing a sintered magnet as in one of claims (9) to (11), wherein calcination or sintering is conducted in an atmosphere having an oxygen partial pressure of more than 0.2 atm.
The composition of the invention comprises a hexagonal Ca series ferrite, to which at least optimum amounts of R and M are added, as shown by the formulae described in the foregoing. By using this composition, excellent magnetic characteristics can be obtained, and at the same time, the temperature characteristics of HcJ can be considerably improved. Furthermore, in the case Co is used as M, while Is is not lowered, rather Is and K1 are simultaneously increased to increase HA, and thus a high Br value and a high HcJ value are realized. Specifically, in the sintered magnet of the invention where Co is used as M, the characteristics satisfying the equations (IV) and (V) above can be obtained at room temperature of about 25xc2x0 C. It has been reported that the conventional Sr ferrite sintered magnet exhibits Br of 4.4 kG and HcJ of 4.0 kOe, but none has been obtained that has HcJ of 4 kOe or more and satisfies the equation (IV). In other words, if HcJ is increased, Br must be low. In the sintered magnet of the invention, although the combination addition of Co and Zn lowers the coercive force lower than the case of the single addition of Co, in some cases lower than 4 kOe, the residual magnetic flux density is considerably increased. At this time, the magnetic characteristics satisfying the equation (V) are obtained. There has been no conventional Sr ferrite sintered magnet having HcJ of less than 4 kOe that satisfies the equation (V). Furthermore, in the invention, the temperature characteristics of HcJ are considerably improved in the case where Ni is added.
The M type ferrite of the invention having a composition where Co is used as M has a saturation magnetization (4xcfx80Is) increased by about 2%, and a crystal magnetic anisotropy constant (K1) or an anisotropic magnetic field (HA) increased by 10 to 20% at most. As the precise measurement of the crystal magnetic anisotropy constant (K1) and the anisotropic magnetic field (HA) is not so easy, there has been no established measurement method, but there can be exemplified a method, in which a torque curve of the anisotropic sample is measured by a torque meter, and then analyzed, to obtain the crystal magnetic anisotropy constants (K1, K2, etc.), a method, in which the initial magnetization curve of the anisotropic sample is measured for the direction of an axis easily magnetized (c axis) and the direction of an axis hardly magnetized (a axis), and the anisotropic magnetic field (HA) is obtained from the point of intersection thereof, and a method, in which the anisotropic magnetic field (HA) is obtained from the differential of second order of the initial magnetization curve in the direction of an axis hardly magnetized (a axis).
When the anisotropic magnetic field HA of the invention is measured by the method, in which the initial magnetization curve of the anisotropic sample is measured for the direction of an axis easily magnetized (c axis) and the direction of an axis hardly magnetized (a axis), and the anisotropic magnetic field (HA) is obtained from the point of intersection thereof, a large value of at least 19 kOe or more, and further 20 kOe or more at most, can be obtained. This becomes an improvement by 10% at most in comparison to the Sr ferrite of the conventional composition.
While the invention exhibits a greater effect of enhancing the HcJ when applied to a sintered magnet, ferrite grains produced according to the invention can be mixed with a binder, such as plastics and rubber, to form a bonded magnet.
The ferrite grains and the sintered magnet of the invention have a small temperature dependency of HcJ, and particularly the ferrite grains of the invention have a considerably small temperature dependency of HcJ. Specifically, the sintered magnet of the invention has a temperature dependency of HcJ within a range of from xe2x88x9250 to 50xc2x0 C. of from xe2x88x925 to 11 Oe/xc2x0 C. (0.23%/xc2x0 C. or less assuming that HcJ at 25xc2x0 C. is 3 kOe), which can be easily reduced to from xe2x88x925 to 5 Oe/xc2x0 C. (0.17%/xc2x0 C. or less assuming that HcJ at 25xc2x0 C. is 3 kOe). The ferrite grains of the invention have an absolute value of a temperature coefficient of HcJ within a range of from xe2x88x9250 to 50xc2x0 C. of 5 Oe/xc2x0 C. or less (0.17%/xc2x0 C. or less assuming that HcJ at 25xc2x0 C. is 3 kOe), which can be easily reduced to 1 Oe/xc2x0 C. or less (0.04%/xc2x0 C. or less assuming that HcJ at 25xc2x0 C. is 3 kOe). It is possible that the temperature coefficient can be zero. Such superior magnetic characteristics under the low temperature environment cannot be attained by the conventional Sr ferrite magnet.
The invention involves a coating type magnetic recording medium having a magnetic layer comprising the ferrite grains dispersed in a binder. The invention also involves a magnetic recording medium having a thin film magnetic layer having the hexagonal magnetoplumbite ferrite phase as similar to the above-described magnet. In these cases, a magnetic recording medium of a high output and a high S/N ratio can be realized owing to the high residual magnetic flux density. Since the magnetic recording medium of the invention can be used as a magnetic recording medium for normal magnetic recording, the recording density can be high. Furthermore, since an absolute value of the temperature coefficient of HcJ can be small, a thermally stable magnetic recording medium can be realized.
A Ba ferrite represented by the following formula:
Ba1xe2x88x92xM3+xFe12xe2x88x92xM2+xO19
is disclosed in Bull. Acad. Sci. USSR, phys. Ser. (English Transl.), vol. 25 (1961), pp. 1405-1408 (hereinafter referred to as Reference 1). In this Ba ferrite, M3+ is La3+, Pr3+ or Bi3+, and M2+ is Co2+ or Ni2+. While it is not clear as to whether Ba ferrite of Reference 1 is powder or a sintered body, this is similar to the Ca ferrite of the invention in the point of inclusion of La and Co. FIG. 1 of Reference 1 shows the change of saturation magnetization depending on the change of x for a Ba ferrite containing La and Co, but in FIG. 1, the saturation magnetization is reduced with the increase of x. Although Reference 1 discloses that the coercive force increases by a few times, there is not disclosure of specific values.
In the invention, on the other hand, by employing the composition, to which the optimum amounts of La and Co, Ni and Zn are added, for the Ca ferrite sintered magnet, the considerable increase of HcJ or the slight increase of Br, and/or the considerable improvement in temperature dependency of HcJ are realized. In the invention, by adding the optimum amounts of La and Co, Ni and Zn to the Ca ferrite particles, the HcJ is greatly increased or its temperature dependency is considerably reduced. It is firstly found in the invention that the combination addition of La and Co, Ni and Zn to a Ca ferrite provides such effects.
A ferrite represented by the following formula:
xe2x80x83La3+Me2+Fe3+11O19
(Me2+: Cu2+, Cd2+, Zn2+, Ni2+, Co2+ or Mg2+)
is disclosed in Indian Journal of Pure and Applied Physics, vol. 8, July 1970, pp.412-415 (hereinafter referred to as Reference 2). This ferrite is similar to the ferrite grains and the sintered magnet of the invention in the point of inclusion of La and Co. However, this ferrite does not contain Ca. In Reference 2, the saturation magnetization as when Me2+ is Co2+ is such low values of 42 cgs unit at room temperature and 50 cgs unit at 0K. While specific values are not disclosed, Reference 2 states that it cannot be a magnet material due to a low coercive force. It is considered this is because the composition of the ferrite of Reference 2 deviates the scope of the invention (the amounts of La and Co are too large).
An isometric hexagonal ferrite pigment represented by the following formula:
Mx(I)My(II)Mz(III)Fe12xe2x88x92(y+z)O19
is disclosed in Japanese Patent Application Kokai No. 62-100417 (hereinafter referred to as Reference 3). In the formula, M(I) is a combination of Sr, Br, a rare earth metal, etc. with a monovalent cation; M(II) is Fe(II), Mn, Co, Ni, Cu, Zn, Cd or Mg; and M(III) is Ti, etc. The hexagonal ferrite pigment disclosed in Reference 3 is similar to the ferrite grains and the sintered magnet of the invention in the point that a rare earth metal and Co are simultaneously contained. However, Reference 3 does not disclose any example in that La, Co and Ca are simultaneously added, and there is no disclosure that the simultaneous addition of them improves the saturation magnetization and the coercive force and provides excellent temperature characteristics of HcJ. Furthermore, in the examples of Reference 3 where Co is added, Ti is simultaneously added as the element of M(III). Because the element of M(III), particularly Ti, functions as an element lowering the saturation magnetization and the coercive force, it is clear that Reference 3 does not suggest the constitution and the effect of the invention.
An optomagnetic recording medium comprising a magnetoplumbite barium ferrite characterized by substituting a part of Ba with La and a part of Fe with Co is disclosed in Japanese Patent Application Kokai No. 62-119760 (hereinafter referred to as Reference 4). This Ba ferrite is similar to the Ca ferrite of the invention in the point of inclusion of La and Co. However, the ferrite of Reference 4 is a material for xe2x80x9coptomagnetic recordingxe2x80x9d in which information is written as a magnetic domain in a magnetic thin film by utilizing a heat effect of light, and the information is read out by utilizing a optomagnetic effect, which is of a technical field different from the magnet and the xe2x80x9cmagnetic recordingxe2x80x9d medium of the invention. Furthermore, in Reference 4, Ba, La and Co are essential in the compositional formula (I), and in the formulae (II) and (III), there is only disclosed that an unidentified tetra-valent metallic ion is added thereto. On the other hand, the ferrite of the invention is the Ca ferrite, in which Ca is essential, and the optimum amounts of La and Co are added thereto, which is different from the composition of Reference 4. That is, as explained with respect to Reference 1, the Ca ferrite of the invention realizes the considerable increase of HcJ and the slight increase of Br, and also realizes the considerable improvement in temperature dependency of HcJ, by using the composition of the Ca ferrite containing the optimum amounts of La and Co. This is firstly realized in the composition of the invention, which is different from Reference 4.
A ferrite magnet having the basic composition represented by the following formula:
(Sr1xe2x88x92xRx)Oxc2x7n((Fe1xe2x88x92yMy)2O3)
(R=La, Nd or Pr, M=Mn, Co, Ni or Zn)
is disclosed in Japanese Patent Application Kokai No. 10-149910. While the addition of CaCO3 is disclosed in the reference, this is a conventional method as a combination addition with SiO2, which aims at xe2x80x9ccontrol of the sintering phenomenon (suppression and acceleration of grain growth). On the other hand, the invention uses Ca as the basic composition of ferrite, which is clearly different from the reference.
A ferrite magnet containing Ca, La and Sr is disclosed in Japanese Patent Application Kokai No. 52-79295. While this is a hexagonal M type ferrite containing Ca along with La as the basic composition, it does not contain Co, Ni, Zn, etc. (element M), which is clearly different from the invention.
In comparison to the above-described conventional ferrite materials, the ferrite material of the invention has the following characteristics.
(1) It exhibits superior characteristics in comparison to the Sr-R-M system (R=La, Nd or Pr, M=Mn, Co, Ni or Zn, disclosed in Japanese Patent Application No. 10-60682) in the point that superior characteristics can be obtained at an ordinary temperature, and the temperature characteristic of HcJ becomes substantially zero.
(2) The above-described temperature characteristics re realized by the characteristics of x=y=0.5 and z=0.85. In the case of the Sr-R-M system and the Ba-R-M ystem, the maximum values of Br and HcJ are obtained at x=y=0.1 to 0.4, but in the case of the Ca-R-M system, Br and HcJ become the maximum when x=y=0.4 to 0.6.
(3) The ferrite grains after calcination are liable to be deformed into a flat form. Therefore, they are suitable as magnetic powder for a mechanically oriented bonded magnet, such as a rubber magnet. In this case, although a flux component, such as barium chloride, has conventionally been added to make the flat shape, this addition can be omitted to render the production cost low.
(4) In the Ca-R-M system, the dependency of HcJ on the atmosphere on the sintering step is large. For example, as demonstrated in the examples described later, HcJ is 3.1 kOe when the sintering is conducted in the air with x=y=0.4, it is greatly increased to 4.2 kOe when the sintering is conducted in oxygen.